1. Field of the Invention
The present invention relates in general to conversion coatings on ceramics, and more particularly to a pack cementation technique to produce conversion coatings on the surface of silicon carbide (SIC).
2. Description of the Related Art
In the past decade, extensive efforts have been made to develop advanced utility power plants and energy conversion/cogeneration systems. Utilization of these advanced systems may significantly improve energy efficiency and reduce toxic emission. To increase the operation efficiency, however, the utility boilers and conversion/cogeneration systems must operate at much higher temperatures and steam pressures. Therefore, many components, such as the heat exchanger tubes and hot-gas clean up systems, will be exposed to corrosive environments at temperatures up to 1100.degree.-1370.degree. C. (2000.degree.-2500.degree. F.). These temperatures are noticeably higher than those experienced in the modern utility boilers. As a result, suitable materials of construction are critical to the success of the high-efficiency systems.
While new high-temperature metallic materials are being developed for these applications, ceramics and ceramic composites are considered the leading candidates to meet the extreme requirements of certain advanced boiler components. Among them, silicon carbide (SIC) is the prime ceramic of interest because of its high-temperature properties, including its excellent thermal conductivity (125 W/mK at RT), low density (3.10 g/cm.sup.3 for dense material), extremely high mechanical strength, relatively good toughness, and low cost. The maximum use temperature of SiC exceeds 1400.degree. C. (2552.degree. F.).
In general, silicon carbide is corrosion resistant to high-temperature environments. However, SiC has not shown satisfactory corrosion resistance to very sulfidizing (reducing) environments. Under reducing atmospheres, active corrosion may occur on the protective SiO.sub.2 scale due to the formation of volatile compounds, such as SiO, SiCl.sub.4, SiCl.sub.2, and SiS. The corrosion rates of SiO.sub.2 can be further escalated by molten ash deposits. When molten ash deposits are present, the SiO.sub.2 formed on the substrate surfaces may be readily destroyed via the well-known fluxing mechanisms. Consequently, the underlying SiC substrates will be constantly exposed to the corrosive environments, and an accelerated wastage of material is observed.
Unlike SiC, chromium carbides have shown much improved corrosion resistance to sulfidation and molten ash deposit. The corrosion resistance is provided by the formation of a protective chromium oxide (Cr.sub.2 O.sub.3) surface scale. The ability of materials to form a Cr.sub.2 O.sub.3 scale has served as the basis for the development of numerous heat-resistant alloys and coatings used in severe environments at high temperatures. For example, chromizing coatings have been used to improve the performance of various boiler components suffering accelerated corrosion attack.
However, chromizing on ceramic materials, such as silicon carbide, its derivatives, and other types of ceramic carbides, has not been reported.